Circuitry to implement DC:DC converters is known in the art. Such circuits receive an input-side DC voltage that is sampled or chopped and transformer-coupled to an output side. On the output side, the waveform is rectified and filtered to provide a regulated output voltage that may be greater than or less than the input voltage. Feedback from output to input can be used to regulate the sampling duty cycle or frequency to provide an acceptably efficient DC:DC converter in a small form factor.
FIG. 1A depicts a so-called voltage-fed push-pull DC:DC converter 10, according to the prior art, having an input side 20 and an output side 30, generally separated by a transformer T1. The input side 20 of the converter is coupled to a source of DC potential Vin. Potential Vin is shown coupled to a pre-regulator 40 whose output potential is controlled within a known tolerance. Although pre-regulator regulator 40 is depicted in the figures, in general it is optional and may be dispensed with if Vin is sufficiently controlled. The output potential from preregulator regulator 40 is sampled or chopped using push-pull switching transistors Q1, Q2 and respective transformer T1 primary windings W1, W2. As best seen in FIG. 1B, a control circuit 50 provides complementary drive signals to the input leads of Q1, Q2 such that when Q1 is on, Q2 is off, and vice versa. Although Q1 and Q2 are shown as switching an end of primary windings W1, W2 to ground potential, it is understood that ground potential implies a stable potential. Stated differently, if desired a potential other than 0 V DC might instead be switchably coupled to an end of primary windings W1 and W2. This understanding that ground is simply a convenient reference potential shall apply throughout this disclosure.
Dual center-tapped secondary transformer windings are shown on output side 30 of DC:DC converter 10, although other winding configurations could instead be used, e.g., a single center-tapped secondary winding could instead be used. Transformer T1's center-tapped secondaries W3-1, W3-2, and W4-1, W4-2 step-up or step-down the chopped waveforms, which are rectified by diodes D1, D2 and capacitor C1, and by diodes D3, D4 and capacitor C2. Other rectification configurations may of course be used, e.g., full-bridge rectification using four diodes. The secondary windings may output different magnitudes Vo1, Vo2 and the number of windings may be greater or less than two. In some configurations, a feedback loop (not shown) may be coupled between the secondary output voltages and control circuit 50.
As shown in FIG. 1B, in an ideal case in which circuit 50 generates drive signals .theta.1 and .theta.2 that are precisely 180.degree. out of phase, switch Q1 will be on when Q2 is off, and vice-versa. As a result, operating efficiency is high, and the filtering requirements on the output side are minimized in that reduction of switching transients will be the primary task of the rectification and filter circuitry. In the configuration shown, output filtering is provided by output capacitors C1 and C2. If desired, inductors could also be used to provide L-C low-pass output filtering. The balanced nature of the output voltage signals and the relative minimal requirements on the output filter are beneficial features of push-pull topography.
But in practice, it is very difficult to provide an inexpensive control circuit 50 that can reliably output two perfectly complementary drive signals .theta.1, .theta.2. If, for example, circuit 50 outputs complementary signals that are slightly out of phase, e.g., where phase shift .DELTA. is non-zero, then there will be times of durations .DELTA. when both Q1 and Q2 are simultaneously on. As a result, operating efficiency will suffer, and more severe switching transients must be filtered from the Vo1, Vo2 signal(s). Thus, much consideration must be given to the design and implementation of a push-pull control circuit 50 to minimize the undesired effects of overlapping drive signals. The result can be a relatively complete control circuit 50 whose component cost can be relatively large when compared to the cost of all components in the overall DC:DC converter. Further, even with an ideal control circuit, body effect diodes are inherently present in Q1 and Q2, and tend to conduct unwanted current, thus decreasing circuit efficiency.
Thus, there is a need for a DC:DC converter topology that provides the efficiency and output filtering advantages associated with a true push-pull configuration, but without requiring a control circuit that can output perfectly complementary drive signals.
The present invention provides such a topology, referred to herein as a pseudo push-pull topography.